The present invention relates generally to solar cells and/or light-to-electrical energy transducers; and, more particularly, to thin layer p-n-type heterojunction solar cells formed from materials selected from the class of I-III-VI.sub.2 chalcopyrite compounds--especially, a p-n-type heterojunction solar cell having a p-type layer of CuInSe.sub.2 and an n-type layer of CdS--and, to methods of manufacture thereof, characterized in that the cells produced have relatively high energy conversion efficiency characteristics--e.g., closely approximating 10%, or greater--are highly stable, and can be formed with low cost fabrication methods on large area, low cost substrates; such characteristics resulting from the formation of a p-n-type heterojunction device wherein the p-type semiconductor layer formed initially comprises a transient p-n-type homojunction formed of a material selected from the class of I-III-VI.sub.2 chalcopyrite compounds in which the p-type region of the transient p-n-type homojunction is formed by vacuum deposition of a copper-enriched ternary composition or the like, and the transient n-type region is formed of a copper-deficient ternary composition or the like, with a layer of low resistivity n-type semiconductor material being vacuum deposited on the transient n-type region of the first semiconductor layer; whereupon interdiffusion of the elemental constituents in the multi-layer structure causes the transient n-type region of the first semiconductor layer to evolve into a p-type region, thereby producing a relatively low resistivity p-n-type heterojunction device essentially devoid of vacancies, voids, copper nodules and the like which tend to decrease conversion efficiencies; and, thereby enabling the formation of large area, thin film solar cells utilizing minimal amounts of critical semiconductor materials to form a low cost, stable, polycrystalline thin-film photovoltaic cell on low cost substrates by the use of low cost fabricating techniques.
Historically, some of the more perplexing problems faced by designers, manufacturers and users of conventional light-to-electrical energy transducers such, for example, as solar cells, have involved the need to improve: (i) the light energy collection efficiency of the cell; (ii) the conversion efficiency of light into electrical energy; and (iii), the cost involved per unit of power generated to produce such cells. Prior to the recent and continuing "energy crisis", research and development efforts have been primarily directed to the first two of the three above-identified factors. As a result, numerous types of solar cells have been designed which have enabled the production of solar cells suitable for use in laboratory experimentation, outer space applications, and the like, wherein the solar cells were relatively small area devices--e.g., on the order of 2".times.2"--generally formed of single crystals which had to be grown and which were relatively expensive. Such crystals are characterized by their lack of grain boundaries; and, are generally limited in size, rarely being larger than about 5" in diameter and, usually, being considerably smaller. However, such devices have been known to achieve relatively high energy conversion efficiencies--sometimes ranging in the order of about 14% to about 16%. While such devices have been highly effective for their intended purposes, their field of practical use is greatly limited; and, they have simply not been satisfactory for generation of power on a practical economic commercial basis.
With the advent of the recent and continuing "energy crisis", efforts of researchers have been redirected; and, a considerable amount of work has been done in attempting to devise various types of energy producing systems which are: (i) environmentally safe; (ii) not constrained by limited natural resources; (iii) devoid of the hazards inherent with nuclear energy generating systems; and (iv), capable of producing sufficient energy to meet mankind's ever-increasing energy requirements on a cost-effective basis which is at least competitive with today's costs for gas, oil, and similar fossil-type fuels or the like. The present invention is believed to constitute the first real step towards attainment of this long sought-after objective.